1. Field of the Invention
The present invention relates to a telephone, and more particularly to a circuit for automatically varying the sound level of a received voice signal in a telephone.
2. Description of Related Art
Many types of automatically variable sound level circuit in a telephone are already known. In these circuits, the sound level of a received voice signal is adjusted in accordance with a noise level inputted from a voice inputting microphone in a telephone. That is, when the level of peripheral noise is so high that it is difficult to hear the transmitted voice of another person, the sound level of the received voice signal is increased in accordance with the peripheral noise level. It is specifically important when communication is performed using a portable phone.
In order to realize such a sound level automatically variable circuit of a received voice signal, there have been some conventional proposals.
FIG. 1 is a block diagram illustrating an example of a conventional sound level automatically variable circuit for the received voice signal which is disclosed in Japanese Laid Open Utility Model disclosure (Zitsukaisho 61-168747). Referring to FIG. 1, in the operation, a noise signal is detected from a voice signal inputted from a microphone 101 by a filter circuit 121. The noise signal amplified by an amplifier 122 is rectified by a rectifying circuit 123. The rectified output voltage is compared with a preset reference voltage in a voltage comparator 124. An analog switch 126 is controlled by the output of the comparator 124 so that the feed-back amount of a variable gain amplifier 110 is varied. As a result, the gain is varied and the sound level of the received voice signal from a speaker 111 is varied.
Further, there is another proposal in which a microphone for inputting noise is provided in addition to a voice inputting microphone and the audio signals from both the microphones are inputted to a comparator to remove a voice signal component of a speaking person such that a noise component can be obtained, as described in Japanese Laid Open Patent Disclosures (Tokukaihei 4-40142 and Tokukaihei 1-202960). As a result, a variable gain amplifier is controlled in accordance with the difference between the two audio signals, i.e., the noise component.
Further, in Japanese Laid Open Patent Disclosure (Tokukaihei 4-82331), a circuit is disclosed in which peripheral noise is calculated from voice signal inputted to a voice inputting microphone using a speech codec, and compared with two threshold levels preset through a button operation by a speaking person before he calls another person or is called by another person such that the sound level of the received voice signal is varied with three levels in accordance with the noise level and the threshold levels.
Furthermore, in Japanese Laid Open Patent Disclosure (Tokukaihei 2-250455), there is disclosed a circuit in which peripheral noise is detected in an off-hook state of a telephone, and the sound level of a speaker is adjusted in accordance with the detected peripheral noise level in a communication state.
In the above-mentioned conventional sound level automatically variable circuits for a received voice signal, however, there are the following problems in the detection of peripheral noise and the gain control of amplification of the received voice signal. That is, in a method according to the Zitsukaisyou 61-168747, since the noise component having the same band as that of the voice signal is removed by the filter in the detection of the peripheral noise, the sound level of the received voice signal cannot be varied even if there is a great noise component in the same band as that of the voice signal. Also, in the methods according to Tokukaihei 4-40142 or Tokukaihei 1-202960, since a microphone different from a microphone for inputting voice is required to detect the peripheral noise, it blocks the need for a small type of telephone such as a portable telephone. In the method according to Tokukaihei 4-82331, since the speech codec section is required to execute DSP processing, there is a problem in making a telephone small, manufacturing it with a low cost, and making it operable with a small current. Further, in the method according to Tokukaihei 2-250455, since the peripheral noise is detected in the on-hook state but it is not detected in the off-hook state, i.e., in the communication state, there is a problem in that the sound level cannot be appropriately adjusted in accordance with the peripheral noise in the communication state in the telephone such as a portable telephone used a place where the peripheral circumstances are varied.